Various type chemical processes are commonly carried out in large, stirred vessels which are frequently provided with auxiliary equipment, such as baffles, condensers, agitators, heat transfer coils which enable heat to be supplied or extracted from the contents of the vessels, and the like. In many cases, however, such processes eventually produce undesirable deposits on the surfaces of the equipment with which the reaction mixtures come into contact including the interior walls of the vessel. Such deposits interfere with the efficient transfer of heat to and from the interior of the vessels. Further, these deposits have a tendency to deteriorate and to partially fragment resulting in contamination of the reaction mixture and the products produced therefrom. This problem is particularly prevalent in polymerization type reactions, since the deposits, or "build-up", of solid polymer on reactor surfaces, not only interfers with heat transfer, but decreases productivity and adversely affects polymer quality.
This problem is particularly bad in the commercial production of polymers and copolymers of vinyl and vinylidene halides, when polymerized alone or with other vinylidene monomers having a terminal CH.sub.2 .dbd.C&lt; group, or with polymerizable monoolefinic monomers. For example, in the commercial production of vinyl chloride polymers, the same are usually produced in the form of discrete particles by polymerization in aqueous suspension systems. When employing such a polymerization system, the vinyl chloride, and other comonomers when used, are maintained in the form of small discrete droplets by the use of suspending agents and agitation. When the reaction is complete, the resultant polymer is washed and dried. These aqueous suspension polymerization reactions are usually conducted under pressure in metal reactors equipped with baffles and high speed agitators. However, these suspension systems are inherently unstable and during the polymerization reaction, vinyl chloride polymer builds up on the interior surfaces of the polymerization reactor, including the surfaces of the baffles and agitator. Obviously, this polymer buildup must be removed since it results in further formation of polymer buildup which in turn results in a crust that adversely affects heat transfer and contaminates the polymer being produced.
Various coatings for reactors have been proposed and several of these have found wide commercial acceptance. The reactor coatings currently used in the polyvinyl chloride industry are primarily phenolic materials such as self condensed phenols or phenols condensed with other materials such as aldehydes or amines. These coatings have proved to be a very significant advancement over the previous methods where no coatings were used. However, experience with such coatings on a commercial scale has shown that more dilute coating solutions would be desirable in order to reduce costs and more importantly to reduce the color of said coating solutions. These prior art coatings are dark colored solutions and as such have a tendency to discolor polymers formed during polymerization. Efforts are continuously being made to find an even more effective reactor coating.
U.S. Pat. No. 4,035,563 teaches using in the aqueous reaction medium salts of nitrous acids such as sodium nitrite together with polyvalent metal salts as a system to reduce "build-up" on the walls of the reactor in a vinyl chloride polymerization. U.S. Pat. No. 4,080,173 teaches using self condensation products of resorcinol as a reactor coating to prevent polymer buildup.